1. Field of the Invention
The present invention relates generally to a digital pulse modulator and, more particularly, to a digital pulse width and position modulator, which accepts a video signal and generates a corresponding video pulse.
2. Description of Related Art
In a scanning system that uses a beam for generating information, a digital pulse forming circuit may be used to control the scanning beam. The beam may vary in intensity and duration according to the pulses used to control the beam.
A laser beam may be used in a printer, for example, for discharging negative image areas on a photoreceptor. The latent electrostatic image formed on the photoreceptor by the beam attracts developing toner in proportion to the latent image charge level in order to develop the image.
As another example, a cathode ray tube uses an electron beam to scan a phosphorous screen. The electron beam may be varied in intensity and duration to accurately display information on the phosphorous screen.
In both examples, a pulse forming circuit may be used to generate pulses to control the intensity and operation time of the respective beams. An apparatus of particular interest to the technology of varying the width and position of pulses used to control a laser beam is disclosed in U.S. Pat. No. 4,965,672 to Duke et al. Another apparatus of general interest that uses pulse number and pulse position modulation to control a laser beam is disclosed in U.S. Pat. No. 4,375,065 to Ohara. Ohara also discloses in U.S. Pat. No. 4,347,523 an apparatus of general interest which uses an input signal to address pulse numbers with corresponding pulse width selection numbers. An image forming apparatus of general interest is disclosed in U.S. Pat. No. 4,905,023 to Suzuki and uses a plurality of conversion tables addressed by an input video image signal to generate pulses.
In a high speed scanning system, the reset time of a pulse forming circuit is the time needed for the pulse forming circuit to reset to an initial state before a new pulse can be generated. Thus, the speed of a pulse forming circuit is limited by the amount of time it takes the circuit to form a pulse and reset to its initial state.
The speed of the pulse forming circuit may be the limiting factor of the speed of the scanning system. In such a case, it is desired to increase the speed of the pulse forming circuit. A solution to this problem is to increase the clock frequency of the pulse forming circuit, however, a higher frequency circuit requires the use of expensive circuit components.